Dual-Curvature Phased Array High-Intensity Focused Ultrasound Transducer for Tumor Therapy

ABSTRACT

A transducer for use with a high intensity focused ultrasound medical system, said transducer comprises a plurality of transducer elements that are made from multi piezocomposite material; said plurality of transducer elements bonded together in turn with an adhesive and said transducer elements deployed along a geometric structure with two curvatures, and a plurality of electrodes arranged on said transducer for exciting said transducer elements in different phases to emit ultrasonic waves in response to an electrical signal applied to said electrodes to form or to be steered to a common focus center in a desired ablation area.

FIELD AND BACKGROUND OF THE INVENTION

Transducers designed for therapeutic ultrasound applications deliver therapeutic power levels through piezoelectric ceramics such as PZT (Lead Zirconate Titanate) or through PZT/polymer composites. The transducer consists of single piezoelectric element or multi elements and electrodes are connected to each piezoelectric element to generate ultrasound waves and control said wave properties such as frequency, amplitude and phase of ultrasound waves. The single element transducer has a fixed focal length that generates a constant focal position while the phased array (multi elements) transducer possesses a focus-steering ability by means of tuning each element's phase, which is called beam forming.

Therapeutic ultrasound is a minimally invasive or non-invasive method to deposit acoustic energy into tissue. The most common therapeutic application for ultrasound transducer is to deliver focused ultrasound or High Intensity Focused Ultrasound (HIFU) to heat and destroy pathogenic tissue or to help drug delivery and release inside the body. Therefore design of therapeutic ultrasound transducer is to deliver acoustic energy through multi-layers of human skin, fat, muscle and soft tissues and all acoustic beams focus at one specific zone under such layers. The precision of focused area and the energy level of such focused point are critical. Moreover, dynamic focusing of the transducer is required to track and target tumors in the moving organ, eg. liver tumors. Therefore, a new HIFU ultrasound transducer is developed by inventors.

OBJECTS OF THE INVENTION

It is an object of this invention to provide a transducer with wide focus-steering ranges in two dimensions.

It is a further object to use a dual-curvature phased array transducer in a HIFU medical system and to provide a method to determine the ratio of radii of two curvatures.

These and other objects of the invention will be understood by those skilled in the art with reference to the following summary and detailed description and the attached drawings.

SUMMARY OF THE INVENTION

A transducer for use with a high intensity focused ultrasound medical system, said transducer comprises a plurality of transducer elements that are made from multi piezocomposite material; said plurality of transducer elements bonded together in turn with an adhesive and said transducer elements deployed along a geometric structure with two curvatures, and a plurality of electrodes arranged on said transducer for exciting said transducer elements in different phases to emit ultrasonic waves in response to an electrical signal applied to said electrodes to form or to be steered to a common focus center in a desired ablation area.

Other and further aspects and features of the invention will be evident from reading the following detailed description of the drawings, which is intended to illustrate, not limit, the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention are described and illustrated herein with reference to the drawings in which like item are indicated by the same reference, in which:

FIG. 1 is a diagram to visualize the principle of focus phased array.

FIG. 2 is a diagram to show a sharp focal zone generated by a flat transducer.

FIG. 3 is a diagram to show a focal zone generated and steered by a flat transducer.

FIG. 4 (a) is a diagram of a spherical transducer.

FIG. 4 (b) is a diagram of a cylindrical transducer.

FIG. 5 is a diagram of a dual-curvature phased array HIFU transducer.

FIG. 6 (a) is a diagram of a spherical transducer with three-dimensional coordination.

FIG. 6 (b) is a diagram of a dual-curvature transducer with three-dimensional coordination.

FIG. 7 (a) is a diagram to demonstrate the intensity.

FIG. 7 (b) is a diagram to demonstrate half beam width.

FIG. 8 (a) is a diagram to show a cross-sectional view in XZ-plane of the simulated acoustic field.

FIG. 8 (b) is a diagram to show a cross-sectional view in YZ-plane of the simulated acoustic field.

FIG. 9 (a) is a diagram to show the steering ability in the depth/Z direction.

FIG. 9 (b) is a diagram to show the steering ability in the depth/Z direction.

FIG. 9 (c) is a diagram to show the steering ability in the depth/Z direction.

FIG. 10 (a) is a diagram to show the steering ability in the X direction.

FIG. 10 (b) is a diagram to show the steering ability in the X direction.

FIG. 10 (c) is a diagram to show the steering ability in the X direction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

When a single element HIFU transducer is employed to deliver energy over a target region, typically a mechanical motion control is used to move the focal zone of said transducer. More advanced design of HIFU transducer used a phased array transducer. Each transducer element in the phased array is a small and independent transducer. Each transducer elements are bonded together in turn with epoxy or other adhesives on a surface. Those transducers are connected to electrodes so that the relative phases of elements in said array could be electrically adjusted. Each element in the array is dynamically adjusted to deliver acoustic wave with different phase. Different phases from different elements create constructive interference of the wave fronts. Therefore delivered energy could be focused at different depths and angles. It means focal zone could be controlled. The principle of phased array is shown in FIG. 1. FIG. 1 shows by using adjusting different phases of each element, an ultrasonic beam is steered to a focal point. Other possible control results are shown in FIG. 2. FIG. 2 shows a sharp focal zone generated by a flat transducer with phased array configuration. A conventional spherical transducer to obtain the desired focal point could achieve same result. However such conventional transducer could not be steered to a specific angle as FIG. 3 shown. Therefore as the piezocomposite technology developed, a spherical transducer or a cylindrical transducer with phased array configuration were constructed. As shown in FIG. 4 (a) and FIG. 4 (b), the phased array transducer with spherical section can steer the focus in the center line; the phased array transducer with cylindrical section has a focus-steering ability in the cylindrical long axis. The advantage of using electrical phase adjustment is to increase the movement control ability of focal location which is one of the critical factors of HIFU system. Dr. Hynynen had used an 1.1 MHz, 256-element spherical phased array HIFU transducer with a 10 cm radius of curvature and a 12 cm diameter generate multi foci simultaneously in a 7.5 mm by 7.5 mm region and also had a dynamic focusing in the depth direction.

In accordance of present invention, Dual-Curvature (DC) phased array HIFU transducer is proposed and the feasibility was proved via the numerical simulations. As shown in FIG. 5, the geometric shape of the DC phased array HIFU transducer (hereinafter referred to as “DC transducer”) is a curved cylindrical. The DC transducer consists of several hundreds of elements and each element is made of piezoelectrical/piezocomposite material. With two radii of curvature, R₁ and R₂, the DC transducer has two natural focal zones, and acoustic beams can converge on two zones when the input electrical phase of all elements is the same. After the small tuning of all elements phase, two foci merge into one focal zone and the focal zone can be steered in the Z or X directions via the further adjustment of phases. With an appropriate arrangement of the DC transducer and patient's torso, the merged focal zone is able to follow the moving target caused by respiration to perform local thermal therapy and/or drug delivery. For instance, X direction aligns to the head-toe direction and Z direction denotes the depth direction.

According to present invention, one embodiment is shown in FIG. 6 (b). FIG. 6 (a) is a cylindrical phased array HIFU transducer with its long axis in Y coordination. Its radius of curvature for every cross section in XZ plane is fixed. For the XY plane cut, the intersection is a line; therefore the radius of curvature is infinite. The DC phased array HIFU transducer as shown in FIG. 6 (b) obtains different results. Its radius of curvature for every cross section in XZ plane and XY plane are not fixed. Therefore the curvature in XY plane also contributes the focusing ability. The geometry structure of the DC phased array is consisted of at least two cylindrical curvatures and combines two features of both cylindrical and spherical phased array HIFU transducers.

More detailed design information to implement the embodiment according to this invention is shown here.

First, the number of elements is determined by taking the ability of focus steering and the cost of the amplifier into account. Theoretically, the more elements are, the better focus steering is. Nonetheless, more elements increase the complexity and cost of the phase and power generator. In the case of the liver tumor therapy, the minimum number of elements is required to make the steering range of the DC transducer sufficient for tracking the moving tumor.

Second, in view of the moving direction and displacement of the liver tumor due to the respiration, more elements are arranged in the X direction of the DC transducer for a wide focus-steering range in the head-toe direction. Additionally, for the channel/cost reduction of the power amplifier, the elements with symmetrical control can be utilized. In this study, the elements in the Y direction are connected in pairs and symmetrically with respect to the X-axial center line of the DC transducer.

Third, in order to avoid skin burn during the HIFU sonications, the wide aperture of the DC transducer is required. Moreover, the surface acoustic intensity defined as the acoustic power of total elements divided by the aperture area restricts the aperture area, and the surface acoustic intensity is determined by the property of piezoelectrical/piezocomposite material.

Fourth, the length, L, height, H, and the radii of two curvatures, R₁, R₂ of the DC transducer as shown in FIG. 5 are related to the focus-steering range in the depth direction, the diameter and length of an ellipsoidal focus, and the spatial averaged intensity at the focus. With the design of two curvatures, the depth-directional focusing of DC transducer can steer in the whole liver. The f#1 is the f number 1 defined as R₁ divided by L, and f#2 is the f number 2 defined as R₂ divided by H. The diameter of the focus is linearly proportional to the f#1 and the length of the focus is linearly related to the f#2 squared. The ratio of R₂ to R₁ is optimized by taking the focus-steering range in the depth direction as well as the spatial averaged intensity at the focus into account.

Here TABLE 1 shows one example of dimension of DC phased array HIFU transducer

TABLE 1 L (mm) 160 H (mm) 100 R₁ (mm) 240 R₂ (mm) 160 The number of element 512 Size of one element (mm) 2.55 by 12.75

It should be noted that among total 512 elements, for one embodiment, 64 elements in the X direction with independent driving and 8 elements in the Y direction with symmetric driving.

To be effective, the absorbed acoustic power in the desired focal point should be greater than a certain amount. A numerical simulation of energy delivering and absorption is beneficiary to the design of the transducer according to this invention. Here by using the Rayleigh-Sommerfeld to integrate the contribution of each point source on the surface of the transducer, the absorbed acoustic power deposition q is given as

$q = \frac{\alpha {p}^{2}}{\rho \; c}$

where α is the ultrasound absorption coefficient of tissue, p is the ultrasonic pressure, ρ is the tissue density, and c is the speed of sound in tissue. Values for α, ρ, c used in simulation are 8.86 Np/m at 1 MHz, 1000 kg/m³, and 1500 m/s. The driving signals for the transducer elements that produce a specific focused pattern are calculated by a pseudo inverse method and the driving frequency is 1 MHz.

The results of simulation as shown in FIG. 7 (a) demonstrate that the intensity is strongest as the CR is 50%, where CR is defined as the ratio of R₂ to R₁, but the Half Beam Width in the depth direction defined as the depth-directional focus-steering range is only 4 cm, as shown in FIG. 7 (b), that is not sufficient for the liver treatment. With the trade-off between the intensity and half beam width in the depth direction, the optimal CR is in the range of 60% to 70% for liver therapy.

FIG. 8 (a) and FIG. 8 (b) illustrate the focusing status of the DC transducer when the electrical phase of each element is the same. All acoustic beams generated by the elements converge on the limited region at two geometric foci. One is at the location, 16 cm away from the center of the DC transducer as shown in FIG. 8 (a), and the focal zone defined as the area of −6 dB peak intensity is parallel to the X axis. The distance between the other focal zone and the center of the DC transducer is 24 cm and the focal zone is perpendicular to the X axis as shown in FIG. 8 (b). After the tuning of each element's phase, two focal zones can merge into one focal zone and the focal zone can steer in the depth/Z direction as shown in FIG. 9 (a), FIG. 9 (b) and FIG. 9 (c). When the focal zone stays at the points (0, 0, 13 cm), (0, 0, 15.5 cm), and (0, 0, 19 cm) where the center of the DC transducer is the original point (0, 0, 0), the maximum intensity at three locations is 6639, 10275, and 5843 W/cm², respectively. These intensities are strong enough to make the liver tumor necrosis or to release the targeted drug. Therefore, the depth-directional focus-steering range of the DC transducer is from 13 cm to 19 cm with respect to the center of the transducer.

Moreover, the X-axial focus-steering ability of the DC transducer was evaluated and the results were shown in FIG. 10 (a), FIG. 10 (b) and FIG. 10 (c). It can be observed that the focusing performance is good during the steering from the point (1 cm, 0, 15.5 cm) in FIG. 10 (a) to the point (3 cm, 0, 15.5 cm) in FIG. 10 (c). The intensity at three points is 12453, 10280, and 7420 W/cm², respectively, which is high enough to ablate the tumor tissue or to activate the anti-tumor drug. Therefore, the DC transducer can steer the focal zone in the x direction from −3 cm to 3 cm with respect to the center of the transducer. 

1. A transducer for use with a high intensity focused ultrasound medical system, said transducer comprising: a plurality of transducer elements that are made from multi piezocomposite material; said plurality of transducer elements bonded together in turn with an adhesive and said transducer elements deployed along a geometric structure with two curvatures, and; a plurality of electrodes arranged on said transducer for exciting said transducer elements in different phases to emit ultrasonic waves in response to an electrical signal applied to said electrodes to form or to be steered to a common focus zone in a desired ablation area.
 2. The transducer of claim 1, wherein said an adhesive is an epoxy.
 3. The transducer of claim 1, wherein said electrical signal controls a frequency and amplitude of said ultrasonic waves.
 4. The transducer of claim 1, wherein said transducer has a better focus-steering ability in one coordination than another coordination.
 5. The transducer of claim 1 has a first section with a curvature radius, R₁ and with a curvature radius, R₂, thereby defining a ratio of R₂ to Rt.
 6. The transducer of claim 5, wherein said ratio is adjusted for optimization according to an intensity of acoustic energy absorption and a focus-steering range in said desired ablation area.
 7. The transducer of claim 1, wherein said transducer contains no Ferromagnetic material so that said transducer could work within magnetic resonance imaging system.
 8. The method to design a dual-curvature transducer for use with a high intensity focused ultrasound medical system, said method comprising the steps of: determining a number of transducer elements for a surface of said dual-curvature transducer by considering a focus steering ability; applying an independent driving method or a symmetric driving method to said transducer elements; determining a wide aperture of said surface of said dual-curvature by considering a necessity of total acoustic power of said dual-curvature transducer, and; determining a ratio of radii of dual curvatures to form a geometric shape of said dual-curvature transducer by considering an intensity of acoustic energy absorption and a focus-steering range in said desired ablation area. 